Emission gas purification system

ABSTRACT

An emission gas purification system is provided capable of making EM clearer and further improving NOx purification capacity. An emission gas purification system  1  is provided for an exhaust system of an engine  3 , and includes a purification unit  16  having a catalyst for purifying NOx according to an air-fuel ratio of an emission gas, and an air-fuel ratio control unit  2  adjusting the air-fuel ratio of the emission gas supplied to the purification unit  16 . The air-fuel ratio control unit  2  purifies the emission gas by setting the emission gas A/F to be near stoichiometric or rich by reducing an intake air mass in an air-fuel mixture.

This application is based on and claims the benefit of priority fromJapanese Patent Application No. 2006-317629, filed on Nov. 24, 2006, andNo. 2007-175980, filed on Jul. 4, 2007 the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an emission gas purification system forpurifying emission gas including nitrogen oxide (NOx).

2. Related Art

Conventionally, an emission gas purification system has been known thathas an oxidation catalyst (DOC, Diesel Oxidation Catalyst) in an exhaustsystem in order to purify an emission gas including soluble organicfractions (SOFs) exhausted from an internal combustion engine. Theemission gas purification system purifies the emission gas by oxidationof SOFs with an oxidation catalyst (DOC) when the air-fuel ratio of theemission gas is lean.

Another emission gas purification system has also been known that has aNOx purification catalyst in an exhaust system of an internal combustionengine (e.g., see Japanese Unexamined Patent Application, FirstPublication 2004-183568). This emission gas purification system purifiessoluble organic fractions (SOFs) and CO when the air-fuel ratio of theemission gas is lean or near stoichiometric, and purifies NOx when theair-fuel ratio of the emission gas is near stoichiometric or rich.

Typically, the purification of NOx with an oxidation catalyst (DOC) iscarried out by controlling the air-fuel ratio of the emission gas to beequal to or below stoichiometric. When the air-fuel ratio is equal to orbelow stoichiometric and the temperature of the oxidation catalyst (DOC)is sufficiently high, it is possible to purify NOx included in theemission gas by supplying a reducing agent to the oxidation catalyst(DOC). However, since the temperature of an emission gas exhausted froma compression ignition internal combustion engine is relatively low, itis difficult to keep the temperature of the oxidation catalyst (DOC)high. As a result, high NOx purification capacity is difficult toobtain.

Furthermore, the purification of NOx with an oxidation catalyst (DOC) iscarried out only when the air-fuel ratio of the emission gas is belowstoichiometric. Consequently, the NOx purification capacity decreaseswhen the air-fuel ratio of the emission gas becomes leaner fromstoichiometric due to transient drive and such.

In contrast, according to an emission gas purification system using aNOx purification catalyst, by oxygen adsorption capacity (OSC), it ispossible to adsorb oxygen to the catalyst when the air-fuel ratio of theemission gas is lean. Accordingly, when the air-fuel ratio of theemission gas is below stoichiometric for purification of NOx, anoxidation reaction for the catalyst is enhanced and the catalysttemperature can be increased. As a result, it is possible to realizehigher NOx purification capacity than in the case where the oxidationcatalyst (DOC) is used.

Moreover, in the emission gas purification system using a NOxpurification catalyst, even when the air-fuel ratio of the emission gasbecomes slightly leaner from stoichiometric, the air-fuel ratio for thecatalyst can be maintained at stoichiometric due to the oxygenadsorption capacity (OSC). Thus, it is possible to prevent the NOxpurification capacity from decreasing.

In view of the above, the present invention aims to provide an emissiongas purification system capable of realizing high NOx purificationcapacity, by providing an NOx purification catalyst in an exhaustemission path and controlling the air-fuel ratio of an internalcombustion engine to be near stoichiometric or rich.

SUMMARY OF THE INVENTION

In order to address to the above problems, an emission gas purificationsystem according to the present invention is provided for the exhaustsystem of a compression ignition internal combustion engine, andincludes: a purification unit provided with a catalyst for purifying NOxaccording to an air-fuel ratio of an emission gas; and an air-fuel ratiocontrol unit for adjusting the air-fuel ratio of the emission gassupplied to the purification unit, wherein the air-fuel ratio controlunit purifies the emission gas by setting the air-fuel ratio of theemission gas to be near stoichiometric or rich, by reducing a chargeefficiency of an air-fuel mixture in a cylinder.

According to the present invention, by decreasing the charge efficiencyof the air-fuel mixture in the cylinder, the emission gas is purified bysetting the air-fuel ratio of the emission gas to be near stoichiometricor rich. Accordingly, it is possible to improve emission gaspurification capacity of the purification unit, thereby making EMclearer and improving NOx purification capacity.

Furthermore, in the emission gas purification system according to thepresent invention, it is preferable that the air-fuel ratio control unitfurther performs at least one of changing of a reflux rate of theemission gas and a throttling.

According to the present invention, by performing at least one of thechanging of the reflux rate of the emission gas and the throttling, itis possible to purify the emission gas by reducing the intake air massin the air-fuel mixture and setting the air-fuel ratio of the emissiongas to be near stoichiometric or rich. Thus, it is possible to select acontrol method accordingly by considering advantages of each method.

Moreover, in the emission gas purification system according to thepresent invention, it is preferable that the purification unit isprovided with a catalyst portion as an active material for purifyingNOx, the catalyst portion containing at least one noble metal selectedfrom the group consisting of Pt, Pd, and Rh, as well as a materialhaving oxygen storage capacity.

At least one noble metal selected from the group consisting of Pt, Pd,and Rh exhibits high purification capacity to the emission gas.Furthermore, the material having oxygen storage capacity serves afunction of a helping catalyst that absorbs the variation in theair-fuel ratio of the emission gas.

In addition, in the emission gas purification system according to thepresent invention, it is preferable that the material having oxygenstorage capacity is a composite oxide containing one of CeO₂ or Ce.

The composite oxide containing one of CeO₂ or Ce can serve a function ofstoring and releasing oxygen. Specifically, while suppressing a decreasein the purification rate of HC and CO by releasing oxygen when rich, itis possible to suppress a decrease in the purification rate of NOx bystoring oxygen when lean.

Furthermore, in the emission gas purification system according to thepresent invention, it is preferable that the active material contains atleast Rh and at least one of Pt and Pd.

Among the at least one noble metal selected from the group consisting ofPt, Pd, and Rh, a noble metal containing Rh as an essential component isparticularly effective in suppressing the decrease in the purificationrate of NOx when rich. Accordingly, when containing any combination ofPt and Rh, Pd and Rh; and Pt, Pd, and Rh as the active material, it ispossible to exhibit particularly high emission gas purificationcapacity.

Furthermore, the catalyst portion of the emission gas purificationsystem according to the present invention preferably contains, as acarrier, at least one porous oxidative product selected from the groupconsisting of Al₂O₃, SiO₂, ZrO₂, and zeolite.

These porous oxidative products have large surface area and are stablein structure. Accordingly, with the catalyst portion having such aporous oxidative product as the carrier, it is possible to exhibit highemission gas purification capacity.

Furthermore, in the emission gas purification system according to thepresent invention, it is preferable that the carrier is a compositeoxide having a perovskite structure.

The composite oxide having a perovskite structure can accept metal ionsat an A site and B site. Accordingly, when using this composite oxide asthe catalyst carrier, it is possible to control the catalyst activity byselecting a type of the metal, etc. Therefore, by using a compositeoxide having the perovskite structure as the catalyst carrier, it ispossible to provide an emission gas purification system having highemission gas purification capacity.

According to the present invention, the air-fuel ratio of the emissiongas is set to be near stoichiometric or rich, and the emission gaspurification capacity by the purification unit is improved, therebymaking EM clearer and improving the NOx purification capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an emission gas purification system accordingto the present invention, along with an internal combustion engine;

FIG. 2 shows a portion of an emission gas purification system;

FIG. 3 shows a simplified diagram of the emission gas purificationsystem as shown in FIG. 1; and

FIG. 4A and FIG. 4B each show diagrams for illustrating the effects ofthe emission gas purification system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment according to the present invention isdescribed with reference to the drawings.

FIG. 1 shows an emission gas purification system 1 according to thepresent invention, along with an internal combustion engine 3(hereinafter referred to as “engine”). The engine 3 (compressionignition internal combustion engine) is, for example, a four-cylindercompression ignition internal combustion engine (only one of thecylinders is shown in the figure) installed in a vehicle (not shown inthe figure).

A combustion chamber 3 c is formed between a piston 3 a and a cylinderhead 3 b of the engine 3. Intake plumbing 4 and an exhaust plumbing 5(exhaust system) are connected to the cylinder head 3 b, and a fuelinjector 6 (hereinafter referred to as “injector”) is attached to thecylinder head 3 b so as to face the combustion chamber 3 c.

The injector 6 is positioned at a central portion of a top wall of thecombustion chamber 3 c, and connected sequentially to a high-pressurepump and a fuel tank (neither are shown in the figure) via a commonline. The fuel consumption amount TOUT, which represents a valve openingtime of the injector 6 is controlled by way of a driving signal from theECU 2 (air-fuel ratio control unit) (see FIG. 2).

Furthermore, a crankshaft 3 d of the engine 3 is provided with amagnetic rotor 30 a, and a crank angle sensor 30 is constituted by thismagnet rotor 30 a and an MRE pickup 30 b. The crank angle sensor 30outputs a CRK signal and a TDC signal as pulse signals to the ECU 2,according to the rotation of the crankshaft 3 d.

The CRK signal is outputted for every predetermined crank angle (30degrees, for example). The ECU 2 derives a revolution speed NE of theengine 3 (hereinafter referred to as “engine revolutions”) based on theCRK signal. The TDC signal is a signal representing that the piston 3 aof each cylinder is at a predetermined crank angle position in thevicinity of top dead center (TDC) when an intake stroke starts, and isoutputted every 180 degrees of crank angle for a four-cylinder typeengine of the present example.

The intake plumbing 4 is provided with a charge compressing device 7.The charge compressing device 7 is provided with the turbocharger unit 8constituted by a turbocharger, an actuator 9 coupled to the turbochargerunit 8, and a vane opening control valve 10.

The turbocharger unit 8 includes rotatable compressor blade 8 a providedfor the intake plumbing 4, rotatable turbine blade 8 b and a pluralityof pivotal variable vane 8 c (only two of these are shown in thedrawing) that are provided for the exhaust plumbing 5, and a shaft 8 dthat integrally couples the compressor blade 8 a and the turbine blade 8b. The turbocharger unit 8 performs a compressing operation, along withthe turbine blade 8 b being driven to rotate by an emission gas in theexhaust plumbing 5, to apply a pressure to intake air in the intakeplumbing 4, by having the compressor blade 8 a, which is integral withthe turbine blade 8 b, be driven to rotate.

The actuator 9 is of a diaphragm type that is actuated by a negativepressure, and is mechanically coupled to each of the variable vane 8 c.The actuator 9 is supplied with a negative pressure via a negativepressure supply path from a negative pressure pump (neither of these areshown in the figure). In the middle of the negative pressure supplypath, the vane opening control valve 10 is provided. The vane openingcontrol valve 10 is constituted of a solenoid valve, and the negativepressure supplied to the actuator 9 varies by controlling the opening ofthe solenoid valve by a driving signal from the ECU 2. With this, acharge pressure is controlled by changing the opening of the variablevane 8 c.

An air-to-water intercooler 11 and a throttle butterfly 12 are providedin the intake plumbing 4 on the downstream side from the turbochargerunit 8, in the stated order from the upstream side. The intercooler 11serves to cool intake air in such a case in which the temperature of theintake air increases due to the compressing operation of the chargecompressing device 7. The throttle butterfly 12 is connected to anactuator 12 a constituted by a direct-current motor, for example. Anopening TH of the throttle butterfly 12 (hereinafter referred to as“throttle butterfly opening”) is controlled by controlling a duty cycleof the current supplied to the actuator 12 a with the ECU 2.

According to one preferred embodiment of the present invention, thecharge efficiency of air-fuel mixture in the cylinder is decreasedwithout throttling by changing the opening of the variable vane 8 cwithout operating the throttle butterfly 12. More specifically, settingthe variable vane 8 c to an open state in accordance with the control bythe ECU 2 according to an operation of an accelerator pedal (not shownin the drawing), without the operation of the throttle butterfly 12decreases a pressure in the charge compressing unit and an intake airmass (decreases the charge efficiency of the air-fuel mixture in thecylinder), and an air-fuel ratio A/F of the emission gas is set so as tobe near an ideal air-fuel ratio (hereinafter referred to as“stoichiometric”) or rich, thereby purifying the emission gas.

Furthermore, although it is not shown in the drawing, the engine 3 isprovided with an intake (IN) valve for taking the air in an intake stepand an exhaust (EX) valve for exhausting the air in an exhaust step. TheIN valve and the EX valve are controlled to be opened and closedaccording to the control of the ECU 2.

Here, increasing an amount of overlap (a time period in which the INvalve and the EX valve are open at the same time) increases the airmass, and reducing the amount of overlap decreases the air mass.Moreover, expanding the amount of overlap decreases a pumping loss.

Thus, the emission gas purification system 1 according to the presentinvention can keep the air-fuel ratio A/F to be rich by controlling theIN valve and the EX valve using the ECU 2, and varying the chargeefficiency to expand the amount of overlap. Furthermore, the change ofthe charge efficiency based on the valve timing of the IN valve and theEX valve does not accompany throttling, and therefore it is possible toreduce the pumping loss, and accordingly, the fuel consumption as well.

Moreover, the intake plumbing 4 is provided with an air flow sensor 31on the upstream side from the turbocharger unit 8. In addition, anintake pressure sensor 32 is provided between the intercooler 11 and thethrottle butterfly 12. The air flow sensor 31 detects an intake air massQA, the intake pressure sensor 32 detects a charge pressure PACT in theintake plumbing 4. These detected signals are outputted to the ECU 2.

Furthermore, an intake manifold 4 a of the intake plumbing 4 ispartitioned into a swirl path 4 b and a bypass path 4 c from anassembled portion to a branched portion. The swirl path 4 b and thebypass path 4 c, respectively, communicate to each of the combustionchamber 3 c via an intake port.

The bypass path 4 c is provided with a swirling device 13 for producinga swirl in the combustion chamber 3 c. The swirling device 13 isprovided with a swirling valve 13 a, an actuator 13 b for opening andclosing the swirling valve 13 a, and a swirl control valve 13 c. Theactuator 13 b and the swirl control valve 13 c are constituted in thesame manner as the actuator 9 and the vane opening control valve 10 ofthe charge compressing device 7, respectively. The swirl control valve13 c is connected to the negative pressure pump. According to the abovedescribed configuration, an opening of the swirl control valve 13 c iscontrolled by a driving signal from the ECU 2 to change the negativepressure supplied to the actuator 13 b, and the change in the opening ofthe swirling valve 13 a controls the degree of the swirl.

In addition, the engine 3 is provided with an EGR device 14 having anEGR tube 14 a and an EGR control valve 14 b. The EGR tube 14 a isconnected between the intake plumbing 4 and the exhaust plumbing 5. Morespecifically, the EGR tube 14 a is connected so as to couple the swirlpath 4 b at the assembled portion of the intake manifold 4 a and theexhaust plumbing 5 on the upstream side of the turbocharger unit 8.Through the EGR tube 14 a, a portion of the emission gas from the engine3 flows back or is refluxed to the intake plumbing 4 as an EGR gas. Withthis, the combustion temperature within the combustion chamber 3 cdecreases, thereby reducing NOx in the emission gas.

The EGR control valve 14 b is constituted by a linear electromagneticvalve attached to the EGR tube 14 a. An amount of the EGR gas iscontrolled by adjusting an amount of valve lift VLACT of the EGR controlvalve 14 b by the driving signal from the ECU 2 that has beenduty-controlled.

Furthermore, the EGR device 14 is provided with an EGR cooling device 15for cooling the EGR gas. The EGR cooling device 15 includes the bypasspath 15 a, an EGR path switching valve 15 b, and an EGR cooler 15 c. Thebypass path 15 a is provided on the downstream side of the EGR controlvalve 14 b of the EGR tube 14 a, so as to bypass the EGR tube 14 a. TheEGR path switching valve 15 b is attached to the branched portion of thebypass path 15 a. The EGR cooler 15 c is attached to the bypass path 15a in the middle thereof. The EGR path switching valve 15 b selectivelyswitches between the side of the EGR tube 14 a and the side of thebypass path 15 a at a portion on the downstream side of the EGR pathswitching valve 15 b according to the control by the ECU 2.

According to the above configuration, when the EGR path switching valve15 b switches to the side of the bypass path 15 a, the EGR gas flows tothe bypass path 15 a to be cooled by the EGR cooler 15 c, and then flowsback to the intake plumbing 4. On the other hand, when the EGR pathswitching valve 15 b switches to the other side, the EGR gas only passesthrough the EGR tube 14 a and flows back to the intake plumbing 4without being cooled.

Here, according to one preferred embodiment of the present invention,controlling the EGR control valve 14 b to change an emission gas refluxrate (EGR rate) without operating the throttle butterfly 12 decreasesthe intake air mass within the air-fuel mixture without throttling. Morespecifically, in accordance with the control of the ECU 2 according tothe operation of the accelerator pedal (not shown in the drawing), theEGR control valve 14 b is set to the open state to increase the emissiongas reflux rate (EGR rate), thereby decreasing the intake air mass. Bythis, the air-fuel ratio A/F of the emission gas is set to be nearstoichiometric or rich in order to purify the emission gas.

Furthermore, a purification unit 16 is provided in the exhaust plumbing5 on the downstream side of the turbocharger unit 8.

Under the atmosphere of stoichiometric, the purification unit 16purifies the emission gas by oxidizing HC and CO in the emission gas andreducing NOx. Moreover, the purification unit 16 stores NOx in theemission gas in an oxidizing atmosphere where a level of oxygen in theemission gas is high. The stored NOx is reduced by a reducing agent inthe emission gas in an oxidizing atmosphere where the oxygen level islow, and thus purified.

The following describes a composition of the catalyst used in thepurification unit 16.

The purification unit 16 includes a material, as an active material thatstores and purifies NOx, that contains at least one noble metal selectedfrom the group consisting of Pt, Pd, and Rh, and that exhibits oxygenstorage capacity (OSC). Moreover, it is preferable to use a compositeoxide containing CeO₂ or Ce as the material that exhibits the oxygenstorage capacity. It should be noted that the material exhibiting theoxygen storage capacity is used in order to maintain a high NOxpurification rate by lessening the change in the oxygen level on asurface of the catalyst when the air-fuel ratio varies from rich tolean.

Furthermore, it is preferable that the active material contains at leastRh, and contains at least one of Pt and Pd. Containing Pt and Pd, asdescribed above, can improve purification capacity for HC and CO on thelean side. In addition, Rh and such exhibit higher reducing capacity forNOx on the rich side or near stoichiometric.

A carrier may further contain at least one porous oxidative productselected from the group consisting of Al₂O₃, SiO₂, ZrO₂, and zeolite. Inaddition, the carrier can be a composite oxide having a perovskitestructure.

Moreover, an amount of Pt, Pd, and Rh supported in total is from 0.1 g/Lto 20 g/L, and preferably from 1 g/L to 10 g/L. Here, when the amount ofPt, Pd, and Rh supported in total is less than 0.1 g/L, the number ofactive sites of the noble metal decreases, and may not exhibit desirablepurification capacity. On the other hand, when the amount supported isgreater than 20 g/L, it is not possible to achieve an improvement inactivity by the amount of noble metals that have increased, resulting inan increase in the cost. Furthermore, the amount of Pt, Pd, and Rhsupported in total is preferably at least 1 g/L considering the coldactivation upon starting up the engine 3, and no more than 10 g/Lconsidering the dispensability of the noble metals in a washcoat.

Moreover, an amount of the composite oxides supported containing CeO₂ orCe in total having oxygen storage capacity is from 1 g/L to 200 g/L, andpreferably from 5 g/L to 150 g/L. Here, when the total amount of thecomposite oxides supported containing CeO₂ or Ce is smaller than 1 g/L,the amount of oxygen to be adsorbed and discharged is too small andatmosphere buffering capacity is not significantly exhibited. On theother hand, when the total amount of the composite oxides supported isgreater than 200 g/L, a catalyst coating layer becomes too thick to besupported, and a pressure drop increases. In addition, the total amountof the composite oxides supported containing CeO₂ or Ce is preferably atleast 5 g/L in order to obtain sufficient atmosphere buffering capacity,and is preferably no more than 150 g/L in terms of ease of support andsuppressing an increase in the pressure drop.

Furthermore, an amount of the porous oxidative products supported isfrom 5 g/L to 300 g/L, and preferably from 10 g/L to 200 g/L. Here, theamount of the porous oxidative products supported smaller than 5 g/L istoo small to support the noble metals in a dispersed manner and causethe noble metals to sinter. On the other hand, when the supported amountof the porous oxidative products is greater than 300 g/L, the catalystcoating layer becomes too thick to be supported, and a pressure dropincreases. Moreover, the amount of the porous oxidative productssupported is preferably at least 10 g/L in order to support the noblemetal in a highly dispersed manner, and is preferably no more than 200g/L in terms of ease of support and suppressing an increase in thepressure drop.

Furthermore, an A/F sensor 33 is provided on the upstream side of thepurification unit 16 of the exhaust plumbing 5. The A/F sensor 33linearly detects an oxygen level A/F in the emission gas in a wide rangeof air-fuel ratios from a rich region to a lean region. The ECU 2calculates an actual air-fuel ratio A/FACT representing an air-fuelratio of an actual gas that has been burned in the combustion chamber 3c, based on the oxygen level A/F detected by the A/F sensor 33. To theECU 2, a detection signal representing an amount of operation(hereinafter referred to as “accelerator position”) AP of theaccelerator pedal (not shown in the figure) is further inputted from anaccelerator position sensor 35.

The ECU 2 is constituted by a microcomputer including an I/O interface,a CPU, RAM, ROM, etc. The detection signals from the various sensors 30to 35 as described above are inputted into the CPU after going throughA/D conversion or trimming at the I/O interface, respectively.

In response to the inputted signals, the CPU determines an drive stateof the engine 3 according to a control program stored in the ROM, etc.Then, the CPU controls the engine 3 including the controls for fuelconsumption amount and the intake air mass, according to the determineddrive state.

Furthermore, the emission gas purification system 1 purifies theemission gas by setting an emission gas A/F to be near stoichiometric orrich by reducing the charge efficiency of the air-fuel mixture in thecylinder. Moreover, the emission gas purification system 1 purifies theemission gas by setting the emission gas A/F to be near stoichiometricor rich by further performing at least one of changing the reflux rateof the emission gas by the EGR control valve 14 b and throttling by wayof the throttle butterfly 12. More preferably, the emission gaspurification system 1 can improve the purification capacity for NOx bythe purification unit 16, while improving fuel consumption by way ofcontrolling the intake air mass to the engine 3 without throttling, andsetting the air-fuel ratio A/F of the emission gas either to be nearstoichiometric or to be on the rich side.

The following describes a method of controlling the intake air mass tothe engine 3, and setting the air-fuel ratio A/F of the emission gaseither to be stoichiometric or to be on the rich side, with reference toFIG. 3, which is a simplified diagram of what is shown in FIG. 1. Below,the like components of FIG. 1 are indicated by the same names andnumerals.

The emission gas purification system 1, as shown in FIG. 3, is providedwith the intake plumbing 4 that introduces air into the engine 3, andthe exhaust plumbing 5 that discharges the emission gas exhausted fromthe engine 3. The intake plumbing 4 is provided with the air flow sensor31, the throttle butterfly 12, and the swirling valve 13 a. The exhaustplumbing 5 is provided with the A/F sensor 33, the variable vane 8 c,and the purification unit 16. Furthermore, the emission gas purificationsystem 1 is provided with the EGR control valve 14 b that refluxes aportion of the emission gas from the engine 3 to the intake plumbing 4as the EGR gas, as well as the turbocharger unit 8 that compresses theintake air in the intake plumbing 4.

Furthermore, the EGR control valve 14 b, the variable vane 8 c, and thethrottle butterfly 12 are opened and closed by the correspondingactuators. The corresponding actuators operate according to the controlof the ECU 2. In addition, the ECU 2 performs the control of theair-fuel ratio of the emission gas supplied to the purification unit 16,by controlling any of the EGR control valve 14 b, the variable vane 8 c,and the throttle butterfly 12.

The following describes a relation between an increase and decrease ofthe air mass, the air-fuel ratio A/F, and the pumping loss according tothe opening and closing operation of each actuator.

When the EGR control valve 14 b is closed in response to the control ofthe ECU 2, the EGR rate decreases and the air mass increases. When theEGR control valve 14 b is opened in response to the control of the ECU2, the EGR rate increases and the air mass decreases. Accordingly, whenthe EGR control valve 14 b is set to the open state, the air-fuel ratioA/F can be set to the rich side. Moreover, since the opening and closingoperation of the EGR control valve 14 b does not involve throttling, itis possible to reduce the pumping loss and the fuel consumption.

Furthermore, when the variable vane 8 c of the turbocharger unit 8 isclosed in response to the control of the ECU 2, the charge pressureincreases and the air mass increases. When the variable vane 8 c of theturbocharger unit 8 is opened in response to the control of the ECU 2,the charge pressure decreases and the air mass decreases. Accordingly,when the variable vane 8 c are set to the open state, the air-fuel ratioA/F can be set to the rich side by changing the charge efficiency.Moreover, since the change in the charge efficiency by the opening andclosing operation of the variable vane 8 c does not involve throttling,it is possible to reduce the pumping loss and the fuel consumption.

Furthermore, the air mass decreases, when the throttle butterfly 12 isclosed in response to the control of the ECU 2. The air mass increases,when the throttle butterfly 12 is opened in response to the control ofthe ECU 2. The throttle butterfly 12 is advantageous in that it is hasexcellent responsiveness because it throttles the intake air directly,and for is able to throttle the air in any drive state.

Moreover, the throttling by the throttle butterfly 12 is not a functioncommonly employed in a compression ignition internal combustion engine.By using the throttling function by the throttle butterfly 12, theemission gas purification system 1 according to the present inventioncan expand the maximum amount of the EGR gas that can be introduced(refer to Reason 1 below), and can improve the response (refer to Reason2 below).

Reason 1

A differential pressure increases between before and after the EGRcontrol valve 14 b by decreasing an internal pressure in the intakemanifold 4 a by throttling (not negative). With this, it is possible tointroduce a greater amount of EGR gas. This exhibits a particular effectin a low load region. In particular, this is an essential function inorder to reduce PM and NOx at the same time by low temperaturecombustion.

Reason 2

The method of changing the charge pressure of the turbocharger unit 8involves a time lag after the variable vane 8 c are opened and closeduntil the revolution speed of the turbine blade 8 b changes.Accordingly, the throttling by the throttle butterfly 12 is superior inresponsiveness.

Furthermore, the method of introducing the EGR gas by the EGR controlvalve 14 b employs lean combustion in which the engine 3 drives on thelean side. In this case, the EGR gas includes air, and time is requiredbefore settling at the intended air mass. Accordingly, the throttling bythe throttle butterfly 12 is superior in responsiveness. Moreover,although it is preferable to decrease the charge pressure in terms withan amount of PM generation in a case in which the intake air mass isreduced in a high load region, closing an intake valve of the throttlebutterfly 12 is faster in response, than opening the variable vane 8 cof the turbocharger unit 8, in order to reduce the charging pressure,and thus advantageous.

Furthermore, examples of a timing at which the air-fuel ratio A/F of theemission gas is set to be stoichiometric or rich (i.e., reducingconditions) to purify the emission gas include, for example, a timing atwhich the temperature of the purification unit 16 and the load of theengine 3 are detected, and the detected temperature of the purificationunit 16 becomes lower than a predetermined temperature, and the detectedload of the engine 3 becomes higher than a predetermined load.

Thus, it is possible to improve the emission gas characteristics bysetting the emission gas A/F to be near stoichiometric or rich (i.e.,reduction condition) when the temperature of the purification unit 16 islower than the predetermined temperature and the load of the engine 3 ishigher than the predetermined load.

As described above, the emission gas purification system 1 purifies theemission gas by reducing the charge efficiency of the air-fuel mixturein the cylinder to set the emission gas A/F to be near stoichiometric orrich. Furthermore, the emission gas purification system 1 can set theemission gas A/F to be near stoichiometric or rich, and improve thepurification capacity of the emission gas by the purification unit 16 byway of performing at least one of the change of the reflux rate of theemission gas by the EGR control valve 14 b and the throttling by thethrottle butterfly 12. Therefore, the emission gas purification system 1can make the EM clearer and further improve the purification of NOx. Inparticular, when the configuration in which the charge efficiency of theair-fuel mixture in the cylinder is changed, and the reflux rate (EGRrate) of the emission gas is changed as needed, it is possible to reducethe pumping loss, as throttling is not involved. Accordingly, it ispossible to improve the emission gas purification capacity whileimproving the fuel consumption. Moreover, emission gas purificationcapacity can be further improved in a case in which a configuration thatexhibits superior purification capacity as described above is applied toa catalyst portion of the purification unit 16.

The present invention is not limited to the embodiment as describedabove, and can be implemented in various ways. Moreover, the presentinvention can be applied to various industrial internal combustionengines other than engines mounted to vehicles such as an engine for aship propulsion apparatus such as an outboard engine in which acrankshaft is provided in the vertical direction. In addition, detailsof the present invention can be modified accordingly within the scope ofthe invention.

WORKING EXAMPLE

FIGS. 4A and 4B show the results of a comparison of NOx purificationrates and catalyst temperatures when a rich spike (fuel rich combustion)is performed and measured for the purification unit 16 of the emissiongas purification system 1 according to the present invention and aconventional purification unit (constituted by an oxidation catalyst(DOC, Diesel Oxidation Catalyst)). For the purification unit 16, acatalyst having an active material of about 10 g/L containing Pt, Pd,and Rh in a ratio of 2:5:1, a complex compound (Ce/Zr) of about 100 g/L,and a porous oxidative product (Al₂O₃) of about 100 g/L was used.Furthermore, for the conventional purification unit (DOC), a catalysthaving about 10 g/L of an active material containing Pt and Pd in aratio of 2:1, and about 100 g/L of a porous oxidative product (Al₂O₃)was used.

As shown in FIG. 4A, the purification unit 16 shows an improvement inthe NOx purification rate compared to the conventional purification unitwhen the rich spike is performed (purification rate of almost 100% inFIG. 4A), and maintains a higher NOx purification rate that than that ofa conventional purification unit during a certain period after thecondition shifted from rich to lean (about 50 to 60 sec in FIG. 4A).

As shown in FIG. 4B, this is because the purification unit 16 canmaintain a high temperature state during a certain period after thecondition shifted from rich to lean (about 20 sec in FIG. 4B) comparedto the conventional purification unit.

As described above, the purification unit 16 of the emission gaspurification system 1 is constituted by the material having oxygenstorage capacity, and therefore the purification unit 16 during the leandrive is in a state in which a certain amount of oxygen is alwaysstored. Therefore, by setting the air-fuel ratio A/F of the emission gasto be rich, the oxidation reaction occurs between the reducing agentthat flows into the purification unit 16 and the oxygen that is storedin the purification unit 16, thereby increasing the temperature of thepurification unit 16 (see FIG. 4B). With this, it is possible toincrease the temperature of the catalyst of the purification unit 16 upto a high temperature state in which high NOx purification capacity canbe obtained, even from an drive state in which the temperature of theemission gas is low (lean state). In other words, it is possible toexpand an drive region in which the NOx purification rate is improved bysetting the air-fuel ratio A/F to be rich.

Moreover, since the purification unit 16 is constituted of a materialhaving oxygen storage capacity, it is possible to maintain the high NOxpurification rate for a certain period of time, even after the air-fuelratio A/F of the emission gas is shifted from rich to lean.

While preferred embodiments of the present invention have been describedand illustrated above, it is to be understood that they are exemplary ofthe invention and are not to be considered to be limiting. Additions,omissions, substitutions, and other modifications can be made theretowithout departing from the spirit or scope of the present invention.Accordingly, the invention is not to be considered to be limited by theforegoing description and is only limited by the scope of the appendedclaims.

1. An emission gas purification system for an exhaust system of acompression ignition internal combustion engine, the system comprising:a purification unit provided with a catalyst for purifying NOx accordingto an air-fuel ratio of an emission gas; and an air-fuel ratio controlunit for adjusting the air-fuel ratio of the emission gas supplied tothe purification unit, wherein the air-fuel ratio control unit purifiesthe emission gas by setting the air-fuel ratio of the emission gas to benear stoichiometric or rich, by reducing a charge efficiency of anair-fuel mixture in a cylinder.
 2. The emission gas purification systemaccording to claim 1, wherein the air-fuel ratio control unit furtherperforms at least one of changing of a reflux rate of the emission gasand a throttling.
 3. The emission gas purification system according toclaim 1, wherein the purification unit is provided with a catalystportion as an active material for purifying NOx, the catalyst portioncontaining at least one noble metal selected from the group consistingof Pt, Pd, and Rh, as well as a material having oxygen storage capacity.4. The emission gas purification system according to claim 2, whereinthe purification unit is provided with a catalyst portion as an activematerial for purifying NOx, the catalyst portion containing at least onenoble metal selected from the group consisting of Pt, Pd, and Rh, aswell as a material having oxygen storage capacity.
 5. The emission gaspurification system according to claim 3, wherein the material havingoxygen storage capacity is a composite oxide containing one of CeO₂ orCe.
 6. The emission gas purification system according to claim 3,wherein the active material contains at least Rh and at least one of Ptand Pd.
 7. The emission gas purification system according to claim 4,wherein the active material contains at least Rh and at least one of Ptand Pd.
 8. The emission gas purification system according to claim 5,wherein the active material contains at least Rh and at least one of Ptand Pd.
 9. The emission gas purification system according to claim 3,wherein the catalyst portion comprises at least one porous oxidativeproduct selected from the group consisting of Al₂O₃, SiO₂, ZrO₂, andzeolite, as a carrier.
 10. The emission gas purification systemaccording to claim 5, wherein the catalyst portion comprises at leastone porous oxidative product selected from the group consisting ofAl₂O₃, SiO₂, ZrO₂, and zeolite, as a carrier.
 11. The emission gaspurification system according to claim 6, wherein the catalyst portioncomprises at least one porous oxidative product selected from the groupconsisting of Al₂O₃, SiO₂, ZrO₂, and zeolite, as a carrier.
 12. Theemission gas purification system according to claim 9, wherein thecarrier is a composite oxide having a perovskite structure.
 13. Theemission gas purification system according to claim 4, wherein thecarrier is a composite oxide having a perovskite structure.
 14. Theemission gas purification system according to claim 6, wherein thecarrier is a composite oxide having a perovskite structure.